Because bridges, pressure vessels and other large steel structures cannot easily be inverted during production, there is no choice but to weld them using the overhead and vertical positions.
In overhead welding, the weight of the molten metal itself tends to cause the weld bead to spread downward. When the first layer is welded, this results in an undercut on the top of the gap (A in FIG. 6), and an improper angle of the junction with the walls on the upper top side of the groove (B in FIG. 6). This can cause defects such as inadequate fusion when the subsequent layer is welded. From the second layer on, the underside of the weld bead will have an improper angle of junction just as in the first layer. Inadequate fusion may result in defective welding of each subsequent layer. This is why such welding is usually executed by means of a semi-automated or hand-welding process which entails a high degree of skill.
Even if this welding process is automated, the welding device must have the same level of capability as a skilled worker who performs the process discussed above. A number of webbing mechanisms and close control of heat will be necessary. Since these are not always enough, the welding process must be conducted at a low temperature with low efficiency. This stands in the way of making the welding process more efficient.
In vertical welding, also, gravity tends to cause the molten metal to spread downward when the open end of the groove is too wide, and the parent material provides inadequate surface tension. The problems are the same as in overhead welding: low heat input and poor efficiency.